🐍 Python📅 Day 8

Lists in Python

Create, index, slice and mutate lists. Common list methods and pitfalls.

🎯

Learning objectives

→Create and modify lists
→Master slicing syntax lst[a:b:c]
→Know the difference between list methods that mutate vs return new

💡 Key points

Lists are ordered, mutable, allow duplicates, can hold mixed types.
Zero-indexed. Negative indices count from the end: lst[-1] is last.
Slicing: lst[start:stop:step] — stop is exclusive.
.append() mutates in place and returns None. Don't do `lst = lst.append(x)`.

💻 Code examples(4)

#1Create and access

python

nums = [10, 20, 30, 40, 50]
print(nums[0])      # 10
print(nums[-1])     # 50
print(len(nums))    # 5

nums[2] = 99        # mutate in place
print(nums)         # [10, 20, 99, 40, 50]

Negative indexing avoids `len(lst) - 1`. Mutation by index is instant.

#2Slicing

python

s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(s[2:5])      # [2, 3, 4]
print(s[:3])       # [0, 1, 2]
print(s[7:])       # [7, 8, 9]
print(s[::2])      # [0, 2, 4, 6, 8]
print(s[::-1])     # reversed

Slices always return a NEW list. s[::-1] is the idiomatic reverse.

#3Common methods

python

lst = [3, 1, 4, 1, 5]

lst.append(9)         # [3, 1, 4, 1, 5, 9]
lst.insert(0, 100)    # [100, 3, 1, 4, 1, 5, 9]
lst.remove(1)         # removes FIRST 1
lst.pop()             # removes + returns last
lst.sort()            # in place
sorted(lst)           # returns new sorted list
lst.reverse()         # in place

sort/reverse mutate. sorted()/reversed() return new. Choose based on whether you need the original.

#4List concatenation vs extend

python

a = [1, 2]
b = [3, 4]

c = a + b             # new list [1, 2, 3, 4]
a.extend(b)           # a is now [1, 2, 3, 4]
a.append(b)           # a is now [1, 2, 3, 4, [3, 4]]  ← nested!

append(list) puts the whole list as one element. extend(list) unpacks it. Different results.

🎯 Practice

Q1. Reverse a list without changing the original.+

reversed_lst = lst[::-1] # or list(reversed(lst))

Q2. Get every 3rd element from a list of 100 numbers.+

nums[::3]

Q3. Sort a list of dicts by the 'age' key.+

sorted(people, key=lambda p: p['age'])

📝 Notes

List comprehensions

The Pythonic way to transform or filter:

nums = [1, 2, 3, 4, 5]
squares = [n * n for n in nums]              # [1, 4, 9, 16, 25]
even_sq = [n * n for n in nums if n % 2 == 0]# [4, 16]

The mutable-default-argument trap

def add_item(x, bucket=[]):    # ❌ default reused across calls
    bucket.append(x)
    return bucket

Two calls in a row share the same list. Fix:

def add_item(x, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(x)
    return bucket